Filters used in liquid filtration can be generally categorized as either fibrous nonwoven media filters or porous film membrane filters.
Fibrous nonwoven liquid filtration media include, but are not limited to, nonwoven media formed from spunbonded, melt blown or spunlaced continuous fibers; hydroentangled nonwoven media formed from carded staple fiber and the like; or some combination of these types. Typically, fibrous nonwoven filter media filters used in liquid filtration have pore sizes generally greater than about 1 micron (μm).
Porous film membrane liquid filtration media is used either unsupported or used in conjunction with a porous substrate or support. Porous filtration membranes have pore sizes smaller than the fibrous nonwoven media, and typically have pore sizes less than about 1 μm. Porous film liquid filtration membranes can be used in: (a) microfiltration, wherein particulates filtered from a liquid are typically in the range of about 0.1 μm to about 10 μm; (b) ultrafiltration, wherein particulates filtered from a liquid, are typically in the range of about 5 nm to about 0.1 μm; and (c) reverse osmosis, wherein particulate matter filtered from a liquid, are typically in the range of about 1 Å to about 1 nm.
Fibrous nonwoven media and porous film membranes are each suitable for use in microfiltration. Microfiltration is widely accepted in industry as a reliable, easily scalable, and benign method to remove microorganisms, such as bacteria, from a fluid stream, and is an essential part of pharmaceutical and biopharmaceutical manufacturing. It is especially important in the biopharmaceutical industry, where microfiltration is used at multiple locations during biopharmaceutical processing.
However, in order to achieve particle retentions equivalent to pore sizes of less than about 1 μm using microfiltration with a fibrous nonwoven media, the number of layers of fibrous material in the filter needs to be increased in order to increase the depth of the nonwoven media. Increasing the number of fibrous layers in the nonwoven media produces both desirable and undesirably results. Increasing the number of fibrous layers produces desirable results by increased tortuosity of a defect path through which a contaminant particle must pass to escape capture by the filter media as well as increasing the contaminant-holding capacity of the filter media. However, increasing the number of fibrous layers in nonwoven media undesirably increases the pressure drop or differential pressure across the media when in use, which translates to increased energy for the filter user and a shorter filter lifespan.
Porous membrane filters used in microfiltration, unlike fibrous nonwoven media, offer a combination of good particle retention, pressure drop and flux, but also tend to be cost-prohibitive, and typically do not provide good contaminant-holding capacity over the entire range of pressure drop, therefore limiting the life of filters using porous membranes.
The two most desired features of a liquid microfiltration membrane are high permeability and reliable retention. Naturally, there is a trade-off between these two parameters, and for the same type of membrane, greater retention has historically been achieved by sacrificing permeability of the membrane. The inherent limitations of the conventional processes for making membranes prevents membranes from exceeding a certain threshold in porosity, and thus limits the magnitude of permeability that can be achieved at a given pore size.
A quantitative measure of microorganism retention by a filtration membrane is customarily expressed as a Log Reduction Value, or LRV. LRV is a logarithm of the ratio of particle concentration in the challenge solution to that in the filter effluent:LRV=Log {[CFU]challenge/[CFU]effluent}
In the case when the filter retains all microorganisms under the conditions of the test, it is customarily to report the LRV as greater than the value obtained when a single microorganism passes the filter. For example, at the challenge particle concentration of 4.77*107 CFU/cm2, the maximum measurable LRV is 8.22. When no particles pass the filter, the LRV is reported as greater than 8.22.
Pore size rating of a membrane is an indicator that the membrane has successfully passed a relevant, standardized bacterial challenge test. The most common pore size rating is 0.22 μm, which is assigned to membranes that pass a Standard Test Method for Determining Bacterial Retention Of Membrane Filters Utilized For Liquid Filtration (ASTM F838-83 test), can be validated to produce sterile effluent after being challenged with ≥107 CFU/cm2 Brevundimonas diminuta. 
Brevundimonas diminuta (ATTC#19146), formerly known as Pseudomonas diminuta, is an aerobic gram-negative bacteria (bacilli). Because of its small size, B. diminuta is a standard microbial organism for validation of membrane filters and the like for sterilization. However, while B. diminuta is representative of most pathogenic bacteria, B. diminuta has proved to be a poor model for a class of microorganisms called Mycoplasma. While representative of most pathogenic bacteria, B. diminuta has proved to be a poor model for a class of microorganisms called Mycoplasma. 
Mycoplasma is a microorganism that can infect cell cultures and can have a substantially deleterious effect to biopharmaceutical manufacturing. The contamination of eukaryotic cell cultures and the like with Mycoplasma is also a common problem, leading to unreliable experimental results and possibly unsafe biological products. This represents a serious problem for manufacturers involved in the development and fabrication of biological and pharmaceutical products. The highly nutritive environment of the media used in cell culture can lead to the propagation of Mycoplasmas, resulting in diminished cell growth as well as the loss of cultures. In contrast to contamination with types of bacteria which can be detected in a short period after infection on the basis of visible effects such as cytopathicity, pH change, abnormal growth, or the media appearing turbid, contamination caused by Mycoplasma may go undetected without noticeable symptoms (Razin, S. 1997. Comparative genomics of Mycoplasmas. Wien Klin Wochenschr 109:551-6. Jung H. Wang S Y, Yang I W, Hsuch D W, Yang W J, Wang T H, Wang: H S. (2003) Detection and treatment of Mycoplasma contamination in cultured cells. Chang Gung Med J. 26: 250-8. Wisher M. (2002) Biosafety and product release testing issues relevant to replication-competent oncolytic viruses, Review. Cancer Gene Ther. 9: 1058-61).
A membrane pore size rating of 0.1 μm indicates that a membrane has been validated to remove Mycoplasma. (See, Roche, K. L.: Levy, R. V., Methods to Validate Microporous Membranes for the Removal of Mycoplasma, BioPharm 1992, 5, (3), 22-33)
For example, membranes having a pore size rating of 0.1 um can be used to filter media, nutrient and cell culture fluid delivered to cells living and growing inside of a bioreactor. Membranes currently exist that have a specific Log Reduction Value (LRV) for A. Laidawii, a test microorganism for Mycoplasma. While it is customarily accepted that LRV>8 is sufficient to claim “full” retention of Mycoplasma, filters having a lower LRV are often used instead in liquid filtration because of greater permeability and higher throughput.
WO/2009/032040, assigned to Millipore Corporation and titled, SERUM-FREE GROWTH MEDIUM FOR ACHOLEPLASMA LAIDLAWII AND METHODS FOR RETENTION TESTING STERILIZING GRADE FILTERS, fully incorporated by reference herein in its entirety, teaches the full retention of Mycoplasma by a filtration medium can be validated to produce sterile effluent after being challenged with ≥1 109× cfu/mL Acholeplasma laidlawii (A. laidlawii; ATCC 23206).
For example, two membranes having a pore size rating of 0.1 μm, Durapore® VV and Express SHR, each available from Millipore Corporation, Billerica, Mass., USA, have Mycoplasma LRVs of 4 and 6, respectively. While Durapore® MV, also available from Millipore Corporation, claims full Mycoplasma retention (LRV>8), it has a lower permeability and capacity in media filtration compared to Durapore® VV and Express SHR.
Synthetic polymers have been formed into webs of very small diameter fibers, i.e., on the order of a few micrometers or less than 1 μm, using various processes including melt blowing, electrostatic spinning and electroblowing. Such webs have been shown to be useful as liquid barrier materials and filters. Often they are combined with stronger sheets to form composites, wherein the stronger sheets provide the strength to meet the needs of the final filter product.
U.S. Patent Publication Number 2004/0038014 issued to Schaefer et al. teaches a nonwoven filtration mat comprising one or more layers of a thick collection of fine polymeric microfibers and nanofibers formed by electrostatic spinning for filtering contaminants. The electrostatic spinning process utilizes an electro spinning apparatus including a reservoir in which the fine fiber forming polymer solution is contained, a pump and an emitter device which obtains polymer solution from the reservoir. In the electrostatic field, a droplet of the polymer solution is accelerated by the electrostatic field toward a collecting media substrate located on a grid. A high voltage electrostatic potential is maintained between the emitter and grid, with the collection substrate positioned there between, by means of a suitable electrostatic voltage source.
The “electroblowing” process is disclosed in World Patent Publication No. WO 03/080905, incorporated herein by reference in its entirety. A stream of polymeric solution comprising a polymer and a solvent is fed from a storage tank to a series of spinning nozzles within a spinneret, to which a high voltage is applied and through which the polymeric solution is discharged. Meanwhile, compressed air that is optionally heated is issued from air nozzles disposed in the sides of, or at the periphery of the spinning nozzle. The air is directed generally downward as a blowing gas stream which envelopes and forwards the newly issued polymeric solution and aids in the formation of the fibrous web, which is collected on a grounded porous collection belt above a vacuum chamber. The electroblowing process permits formation of commercial sizes and quantities of nanowebs at basis weights in excess of about 1 gsm, even as high as about 40 gsm or greater, in a relatively short time period.
U.S. Patent Publication Number 2007/0075015 issued to Bates et al. teaches a liquid filtration media including at least one layer of nanofibers having average diameters less than 1,000 nanometers optionally disposed on scrim layer for filtering particulate matter in a liquid. The filtration media have flow rates of at least 0.055 L/min/cm2 at relatively high levels of solidity. The media apparently has non-diminishing flow rates as differential pressures increase between 2 psi (14 kPa) and 15 psi (100 kPa).
U.S. Patent Publication Number 2007/0018361 issued to Xu teaches fabricating nanofibers by reactive electrospinning, wherein the electrospinning process is coupled with an in-line reactor where chemical or photochemical reactions take place. The processes taught in Xu use electrospinning to allow for the production of nanofibers from crosslinked polymers and other materials.
U.S. Patent Publication Number 2009/0026137 issued to issued to Chen teaches fabricating liquid filter with a composite medium that has a nanoweb adjacent to and optionally bonded to a microporous membrane. The membrane is characterized by an LRV value of 3.7 at a rated particle size, and the nanoweb has a fractional filtration efficiency of greater than 0.1 at the rated particle size of the membrane. The nanoweb also has a thickness efficiency ratio of greater than 0.0002 at that efficiency. The nanoweb acts to provide depth filtration to the membrane.
It would be desirable to have a reliable electrospun nanofiber liquid filtration medium having a microorganism LRV greater than about 8, suitable for full retention of microorganisms such as bacteria, Mycoplasma in particular, and/or a B. Diminuta LRV greater than about 9, suitable for full retention of B. Diminuta, when removed from a liquid passing through the filtration medium, while simultaneously achieving high permeability and high throughput.
Additionally, the porous electrospun nanofiber filtration medium would be readily scalable, adaptable to processing volumes of sample fluids ranging from milliliters to thousands of liters, and capable of use with a variety of filtration processes and devices. The invention is directed to these, as well as other objectives and embodiments.